A data processing system including multiprocessors is able to carry out several processes at the same time. To enable a utilization system to manage files of processes awaiting implementation, each process has a priority level allocated to it; the highest priority process is the process having the lowest order number, i.e., a zero order number.
While they are being performed, the processes derive demands for service. These demands for service are made either on another process or to the process itself. The demands are referred to as asynchronous requests or traps to indicate that they divert the process in question from its normal development.
When an asynchronous trap is derived by a process, this process (an applying process) is supplied with a procedure that implements the demanded service. The applying process consequently seeks a required process amongst the processes being developed in the processing system. If the required process is in active service, the applying process supplies the required process with a serivce implementation signal and thereby diverts the required process from its normal development.
This processing method has disadvantages. In actuality, if the required process was in a critical stage or section of development, the system is placed in a special situation. The process is said to be in a critical phase when the portion of the program being implemented is located in a critical section, i.e., that this implementation mobilizes a critical resource. A resource is defined as critical if it is the only resource for the system; the resource may only be accessed by one device at a time. At the same time a process takes possession of a critical resource it suspends access to the resource by other processes as long as the process considers the possession to be necessary. If a process of this kind is required, this suspension does not cause the cancellation of the reservation and the resource continues to be inaccessible to the other processes. For example, this situation may cause paralysis of the system if the critical resource is necessary to continue the operation of the other processes.